Copper Vacancies and Heavy Holes in the Two-Dimensional Semiconductor KCu_3–xSe₂

Abstract

The two-dimensional material KCu3–x Se2 was synthesized using both a K2Se3 flux and directly from the elements. It crystallizes in the CsAg3S2 structure (monoclinic space group C2/m with a = 15.417(3) Å, b = 4.0742(8) Å, c = 8.3190(17) Å, and β = 112.94(3)°), and single-crystal refinement revealed infinite copper-deficient [Cu3–x Se2]− layers separated by K+ ions. Thermal analysis indicated that KCu3–x Se2 melts congruently at ∼755 °C. UV–vis spectroscopy showed an optical band gap of ∼1.35 eV that is direct in nature, as confirmed by electronic structure calculations. Electronic transport measurements on single crystals yielded an in-plane resistivity of ∼6 × 10–1 Ω cm at 300 K that has a complex temperature dependence. The results of Seebeck coefficient measurements were consistent with a doped p-type semiconductor (S = +214 μV K–1 at 300 K), with doping being attributed to copper vacancies. Transport is dominated by low-mobility (on the order of 1 cm2 V–1 s–1) holes caused by relatively flat valence bands with substantial Cu 3d character and a significant concentration of Cu ion vacancy defects (p ∼ 1019 cm–3) in this material. Electronic band structure calculations showed that electrons should be significantly more mobile in this structure type.

High Hole Mobility and Nonsaturating Giant Magnetoresistance in the New 2D Metal NaCu4Se4 Synthesized by a Unique Pathway

“…xSe2 (~7.1 m0) 5 , the value of m* in NaCu4Se4 is much smaller, which is consistent with the much higher mobility.…”

“…The low-melting flux synthesis method, which gives access to intermediate temperatures, has been proven a powerful approach to discover new phases in the A/Cu/Q system. [18][19] Despite the great success of the polychalcogenide flux method in enabling productive synthesis routes to new materials, e.g., KCu3-xSe2, 5 NaCu4Se3, 12 and NaCu6Se4, 15 in rare instances we also discover that the nature of the precursors can also be critical in forming new phases. For example, the use of Cu2O was shown to be unique in obtaining the mixed-anion oxychalcogenide compound Na1.9Cu2Se2•Cu2O which features metallic behavior with mixed valency.…”

α-CsCu5Se3: Discovery of a Low-Cost Bulk Selenide with High Thermoelectric Performance

“…The κ lat value of α-CsCu 5 Se 3 is only 0.8–0.4 W/(m K), especially above 600 K it is even lower than that of Cu 2 Se. The inset of Figure d reveals that α-CsCu 5 Se 3 exhibits a thermal transport behavior of the crystalline solid, showing a strong temperature dependence of κ lat ∝ T –1 that is dominated by a phonon–phonon Umklapp scattering mechanism . The thermal transport behavior of α-CsCu 5 Se 3 is thus totally different from the “liquidlike” behavior of Cu 2 Se .…”

KCu7P3: A Two-Dimensional Noncentrosymmetric Metallic Pnictide

“…xP, 40 CaCu4P2, 14 KCu3-xSe2 41 and NaCu4Se3. 42 Significant disorder of both Cu and P atoms was further evidenced by the need to invoke site splitting of Cu4 (Figure 2b) and the addition of EADP and ISOR constraints to obtain physical values for the ADPs of these species.…”